Solid viscosity index improvers which provide excellant low temperature viscosity

ABSTRACT

A solid, cyclone-finishable polymer which does not exhibit cold flow and provides an unexpected balance of viscosity improving properties for oil compositions is produced by hydrogenating a copolymer comprising a homopolymer block of isoprene connected to a copolymer block of isoprene and a monoalkenyl aromatic hydrocarbon. The amount of the monoalkenyl aromatic hydrocarbon is sufficient to allow cyclone finishing although sufficiently distributed to provide oil compositions having good low temperature viscosity. Prior to hydrogenation, the copolymers have a number average molecular weight between 125,000 and 275,000, preferably 150,000 to 240,000, and a total monoalkenyl aromatic content between 10 and 2 percent by weight, preferably between 7 and 3 percent. The copolymer is hydrogenated to substantially saturate the isoprene and to reduce the size of monoalkenyl aromatic segments in the copolymer block.

BACKGROUND

This invention relates to polymers useful as a viscosity index improverand to oil compositions compromising the same. More particularly, thisinvention relates to hydrogenated copolymers of isoprene and amonoalkenyl aromatic hydrocarbon.

As is well known, the viscosity of lubricating oils varies withtemperature, and it is important that the oil not be too viscous (thick)at low temperature nor too fluid (thin) at higher temperature. Thevariation in the viscosity-temperature relationship of an oil isindicated by the so-called viscosity index (VI). The higher theviscosity index, the less the change in viscosity with temperature. Ingeneral, the viscosity index is a function of the oils viscosity at agiven lower temperature and a given higher temperature. The given lowertemperature and the given higher temperature for lubricating oils havevaried over the years but are fixed at any given time in an ASTM testprocedure (ASTM D2270). Currently the lower temperature specified in thetest is 40° C. and the higher temperature specified in the test is 100°C.

The thickening efficiency of a polymeric VI improver is an important,and frequently, the principal consideration in its selection for use inoil compositions. Polymeric VI improvers which significantly increasethe high temperature kinematic viscosity without appreciably increasingthe low temperature kinematic viscosity are preferred. The thickeningefficiency of any given polymeric VI improver will vary with polymercomposition and structure but will increase with increased molecularweight. Other properties are important including the ability of the VIimprover to maintain an increase in viscosity even after subjected tomechanical shear; the high temperature, high shear rate (HTHSR)viscosity response of an oil composition containing the viscosity indeximprover; the low temperature viscosity response of an oil containingthe viscosity index improver; the engine pumpability of a lubricatingoil composition containing the viscosity index improver; and the lowtemperature startability of an engine containing the lubricating oilcomposition. It should be noted that viscosity index improvers which aresolid and do not exhibit cold flow are particularly preferred for easein packaging and handling. Polymers of this type are usually, but notalways, capable of being separated from solvent during manufacture bymeans of cyclone-finishing techniques.

Block copolymers comprising a single polymeric block of a monoalkenylaromatic hydrocarbon compound and a single polymeric block of isopreneoffer a good balance of viscosity index improver properties as taught inU.S. Pat. No. 3,772,196. However, these polymers comprise relativelyhigh monoalkenyl aromatic hydrocarbon contents which give lower HTHSRresponses than polymers having similar molecular weights and less of themonoalkenyl aromatic hydrocarbons.

A viscosity index improver having a relatively small amount ofmonoalkenyl aromatic hydrocarbon is taught in U.S. Pat. No. 3,775,329which describes tapered copolymers of isoprene and monoalkenyl aromatichydrocarbon monomers. Although the patent excludes the use ofrandomizers, it is asserted that the tapered copolymers do not havesignificant homopolymer blocks of isoprene or the monoalkenyl aromatichydrocarbons. The described VI improvers include non-cyclone finishableVI improvers, VI improvers which provide relatively low HTHSR responsesin multigrade oils, and VI improvers with low thickening efficiencies.

U.S. Pat. No. 4,418,234 describes diene block copolymers which have highvinyl contents and provide good high shear rate responses. For anymolecular weight, an increase in vinyl content lowers the thickeningefficiency of the VI improver. The patent asserts that randomizers canbe used as long as the total amount of homopolymer blocks of monoalkenylaromatic hydrocarbon is less than about 5 percent by weight, preferablyless than about 2 percent.

SUMMARY OF THE INVENTION

A solid, cyclone-finishable polymer which does not exhibit cold flow andprovides an unexpected balance of viscosity improving properties for oilcompositions is produced by hydrogenating a copolymer comprising ahomopolymer block of isoprene connected to a copolymer block of isopreneand a monoalkenyl aromatic hydrocarbon. The amount of the monoalkenylaromatic hydrocarbon is sufficient to allow cyclone finishing althoughsufficiently distributed to provide oil compositions having good lowtemperature viscosity. Prior to hydrogenation, the copolymers have anumber average molecular weight between 125,000 and 275,000, preferably150,000 to 240,000, and a total monoalkenyl aromatic content between 10and 2 percent by weight, preferably between 7 and 3 percent. Thecopolymer is hydrogenated to substantially saturate the isoprene and toreduce the size of monoalkenyl aromatic segments in the copolymer block.

DETAILED DESCRIPTION OF THE INVENTION

A cyclone-finishable polymer useful for improving the viscosity index ofa lubricating oil is produced by anionically polymerizing a copolymerhaving a number average molecular weight between 125,000 and 275,000,from 90 to 98 percent by weight of isoprene, from 10 to 2 percent byweight of a monoalkenyl aromatic hydrocarbon, and a structure of A--A/Bwherein A is a homopolymer block of the hydrogenated isoprene and A/B isa copolymer block of the hydrogenated isoprene and a monoalkenylaromatic hydrocarbon. At least 85% of the isoprene units, preferably atleast 93%, have the 1,4-configuration and the homopolymer block of theisoprene comprises at least 60% of the copolymer by weight. Thecopolymer is hydrogenated to saturate at least 95% of the isoprene unitsand at least 5% of the monoalkenyl aromatic hydrocarbon units.

The polymers of this invention are cylcone-finishable as a solid polymercrumb and do not exhibit cold flow. Oil compositions containing thepolymers have a good balance between high temperature, high shear rateviscosity response; low temperature viscometric response; engine oilpumpability and low temperature startability.

The block copolymer useful as a VI improver in the present invention maybe prepared in at least two different ways. The polymerization may becompleted by placing a desired amount of isoprene into a reaction vesseland, after polymerizing about 60 to about 90% of the polymer as ahomopolymer block, the desired amount of monoalkenyl aromatichydrocarbon monomer may then be added. In the absence of a randomizingagent, the mixture of isoprene and monoalkenyl aromatic hydrocarbon addsa tapered copolymer block to the initial isoprene block. Processconditions for making tapered copolymer blocks are the same as formaking tapered copolymers which are described in U.S. Pat. No. 3,775,329which is incorporated by reference herein. Alternatively, polymerizationmay be initiated by placing the amount of isoprene to be incorporatedinto the homopolymer isoprene polymer block in a reaction vessel andallowing the isoprene to react to substantial completion before addingisoprene and monoalkenyl aromatic hydrocarbon monomer in the absence orrandomizing agents to polymerize the copolymer block. In either case,addition of the monoalkenyl aromatic hydrocarbon late in thepolymerization results in a large homopolymer block of isopreneconnected to a copolymer block having sufficient segments of themonoalkenyl aromatic hydrocarbon to be cyclone-finishable. Polymershaving higher total molecular weights generally require less amounts ofthe monoalkenyl aromatic hydrocarbon to be cyclone-finishable as apolymer crumb.

Prior to hydrogenation, the copolymers have a number average molecularweight between 125,000 and 275,000, preferably between 150,000 and240,000. The hydrogenated isoprene homopolymer blocks have numberaverage molecular weights within the range from about 75,000 to about250,000, preferably 90,000 to 225,000. Molecular weight as used hereinis the molecular weight as determined from the peak value using GPCtechniques.

U.S. Pat. No. 3,772,196 which is incorporated by reference hereinteaches the use of an organo metallic compound, particularly lithiumcompounds, to prepare isoprene polymers having high 1,4 contents andthese compounds are particularly preferred for use in preparing theisoprene polymer which is useful in the present invention. Suitableorgano metallic compounds containing one or more lithium atoms include,generally, compounds satisfying the general formula RLi_(n) wherein nmay be 1 or 2. Suitable organo metallic compounds containing a singlelithium atom which are useful in preparing the viscosity index improverof the present invention include, but are not limited to, compoundswherein R is unsaturated such as allyl lithium, methallyl lithium andthe like; compounds where R is aromatic such as phenyl lithium, tolyllithium, zyllyl lithium, naphthyl lithium and the like; and compoundswherein R is an alkyl unit such as methyl lithium, ethyl lithium, propyllithium, butyl lithium, amyl lithium, hexyl lithium, 2-ethyl hexyllithium, n-hexadecyl lithium and the like. Secondary-butyl lithium is amost preferred initiator for use in preparing the VI improver of thepresent invention.

The polymerization to prepare the VI improver of the present inventionwill be completed in a suitable solvent useful in the preparation ofblock copolymers containing a conjugated diolefin and a monoalkenylaromatic hydrocarbon monomer. Suitable solvents include, generally,hydrocarbon solvents such as paraffins, cycloparaffins, alkylsubstituted cyclo paraffins, aromatics and alkyl substituted aromaticscontaining from about 4 to about 10 carbon atoms per molecule. Suitablesolvents include, but are not limited to, benzene, toluene, cyclohexane,methylcyclohexane, n-butane, n-hexane, n-heptane, and the like.Cyclohexane is preferred.

Suitable monoalkenyl aromatic hydrocarbon monomers useful for preparingthe VI improvers of this invention include, but are not necessarilylimited to, styrene, various alkyl-substituted styrenes, alkoxysubstituted styrenes, vinyl naphthalene, alkyl-substituted vinylnaphthalenes and the like. Of these, styrene is particularly preferred.

Preparation of the block copolymer useful as the VI improver of thisinvention may be completed at a temperature broadly, within the rangefrom about 20° C. to about 100° C., preferably at a temperature withinthe range from about 50° C. to about 70° C. The copolymerizationreaction is carried out in an inert atmosphere, preferably under anitrogen blanket, and the polymerization will be carried out, generally,under pressure, for example, at a pressure within the range from about0.5 to about 10 bars. The concentration of initiator during thepolymerization may vary over a relatively wide range but will becontrolled in combination with the monomer concentration so as toproduce blocks within the polymer having the desired molecular weight.

The block copolymer useful as a VI improver in this invention ishydrogenated at conditions sufficient to hydrogenate at least 95% of theisoprene units and at least 5% of the monoalkenyl aromatic hydrocarbonunits. Preferably, from 10 to 20% of aromatic unsaturation ishydrogenated. Hydrogenation at these levels is very reproducible forcopolymers having from 10 to 2% by weight of monoalkenyl aromatichydrocarbon units as determined by NMR integration of the aromaticprotons.

A particular preferred method for selectively hydrogenating the blockcopolymer useful in this invention is described in U.S. Pat. No.3,700,633, the disclosure of which patent is herein incorporated byreference. In the process taught in U.S. Pat. No. 3,700,633hydrogenation of the block copolymer is accomplished in the same solventas was used during the polymerization using a hydrogenation catalystcomprising the reaction product of an aluminum alkyl and a nickel orcobalt carboxylate or alkoxide. In general, hydrogenation isaccomplished at a temperature within the range from about 25° C. toabout 175° C. at a hydrogen partial pressure of at least 50 psig, andusually at a hydrogen partial pressure within the range from about 250to about 1500 psig. In general, contacting times within the range fromabout 5 minutes to about 8 hours will be sufficient to permit thedesired degree of hydrogenation. In general, the selectivelyhydrogenated block copolymer may be recovered as a crumb using knowntechniques.

The selectively hydrogenated block copolymer of this invention may beadded to a variety of oils including crude oil, mineral and syntheticoils, lubricating oils, diesel oils, hydraulic oils, automatictransmission oils and the like. In general, the concentration of theselectively hydrogenated block copolymer in such oils may vary betweenwide limits with amounts within the range from about 0.2 to about 3weight percent being most common. Concentrations within the range fromabout 0.4 to about 2 weight percent are preferred and concentrationswithin the range from about 0.5 to about 1.5 weight percent are mostpreferred. Lubricating oil compositions prepared with the selectivelyhydrogenated block copolymer of this invention may also contain otheradditives such as anti-corrosive additives, anti-oxidants, detergents,pour point depressants, anti-wear/extreme-pressure agents, one or moreadditional VI improvers and the like. Typical additives which are usefulin the lubricating oil compositions of this invention and theirdescription will be found in U.S. Pat. Nos. 3,772,196 and 3,835,083, thedisclosure of which patents are herein incorporated by reference.

The preferred viscosity index improver prior to hydrogenation has thestructure A--A/B--B wherein the number average molecular weight of thecopolymer is between 150,000 and 240,000, the styrene content is between7% and 3%, the homopolymer block of isoprene comprises at least 75% ofthe copolymer by weight, and B is a homopolymer block of styrene havinga number average molecular weight of at least 4,000, most preferably atleast 6,000.

The preferred polymers are anionically polymerized in a cyclohexanesolution until the solution contains 15 to 50 weight percent of thepolymer. Polymerization occurs under a nitrogen blanket at a nitrogenpartial pressure within the range from about 0.5 to about 2 bar.

After the polymerization reaction is completed, the preferred copolymersare hydrogenated in the presence of a catalyst prepared by combiningtriethyl aluminium and nickel 2-ethylhexanoate. The hydrogenation willmost preferably reduce the saturation of the isoprene to less than 98%of the ethylenic unsaturation originally contained in the copolymer andsaturate from 10% to 30% of the styrene to give a total styrene contentbetween 6 and 4% by weight for polymers having a number averagemolecular weight between 190,000 and 210,000.

Having thus broadly described the present invention and a preferredembodiment thereof, it is believed that the invention will become evenmore apparent by reference to the following examples. It will beappreciated, however, that the examples are presented solely forpurposes of illustration and should not be construed as limiting theinvention unless a limitation introduced specifically in the examples isalso incorporated specifically into the claims appended hereto.

EXAMPLES

In the following examples, copolymers described and claimed in thisapplication were made and compared with known copolymers to establishimproved properties and performance as a VI improver. All of thecopolymers were formulated into SAE 10W-40 and SAE 5W-30 multigradelubricating oil compositions. The copolymers within the scope of thepresent invention were cyclone-finishable as a solid polymer crumb andexhibited good viscometric properties.

EXAMPLE 1

Living isoprene blocks were prepared by anionically polymerizing about85% of an initial charge of 67.39 lbs. of isoprene in cyclohexane. Thepolymerization reaction was promoted by sec-butyllithium in the absenceof any randomizing compounds. The living isoprene blocks had at least93% 1,4-polymerized isoprene units. Polymerization continued after theaddition of 4.64 lbs. of styrene until the number average molecularweight of the copolymer was 189,100 as measured by GPC. The copolymerhad a polystyrene content of 6.5% by NMR and a total molecular weight ofthe thermal polystyrene blocks of 7,800 as measured by GPC on the solidmaterial recovered after ozone degradation of the polymer prior tohydrogenation.

The copolymer was hydrogenated with a catalyst composition comprisingnickel and triethyl aluminum with Al/Ni ratio from 2.1 to 2.5. Thecatalyst composition was prepared as a masterbatch and used for all ofthe examples. Hydrogenation also reduced the polystyrene content to5.1%. The copolymer was recovered as a solid polymer crumb which did notexhibit cold flow.

The hydrogenated copolymer was added to an oil composition whichcontained 9.1% by weight of an experimental additive package, 0.3% Hitec623 which is a pour point depressant, and the balance was HVI 100N oil.The oil composition contained an amount of the copolymer effective togive a kinematic viscosity of 11 centistokes at 100 C and a SAE grade of5W-30.

The hydrogenated copolymer was also added to oil compositions whichcontained 31% by weight HVI 250N (DP) oil, 7.75% by weight of anadditive package sold as Lubrizol 7573A, 0.3% Acryloid 160 pour pointdepressant, and the balance was HVI 100N oil. The oil compositioncontained an amount of the copolymer effective to give a kinematicviscosity of 14 centistokes at 100° C. and an SAE grade of 10W-40.

The low temperature viscosity of the oil compositions were measuredaccording to ASTM D-4684 and the results are reported in the followingTable for comparison with subsequent examples.

EXAMPLE 2

Living isoprene blocks were prepared by anionically polymerizing about85% of an initial charge of 14.76 lbs. of isoprene in cyclohexane. Thepolymerization reaction was promoted by sec-butyllithium in the absenceof any randomizing compounds. The living isoprene blocks had at least93% 1,4-polymerized isoprene units. Polymerization continued after theaddition of 1.02 lbs. of styrene until the number average molecularweight of the copolymer was 201,400. The copolymer had a polystyrenecontent of 6.2% by NMR and a total molecular weight of 9,100 as measuredby GPC according to Example 1.

The copolymer was hydrogenated as described for Example 1. Hydrogenationreduced the polystyrene content to 5.4%. The copolymer was recovered asa solid polymer crumb which did not exhibit cold flow.

The hydrogenated copolymer was added to oil compositions as described inExample 1. The low temperature viscosity of the oil compositions weremeasured according to ASTM D-4684 and the results are reported in thefollowing Table.

EXAMPLE 3

Living isoprene blocks were prepared by anionically polymerizing about85% of an initial charge of 18.71 lbs. of isoprene in cyclohexane. Thepolymerization reaction was promoted by sec-butyllithium in the absenceof any randomizing compounds. The living isoprene blocks had at least93% 1,4-polymerized isoprene units. Polymerization continued after theaddition of 1.27 lbs. of styrene until the number average molecularweight of the copolymer was 231,000. The copolymer had a polystyrenecontent of 6.4% and the total molecular weight of block polystyrene was10,800 as measured by GPC prior to hydrogenation according to Example 1.

The copolymer was hydrogenated as described for Example 1. Hydrogenationreduced the polystyrene content to 4.5%. The copolymer was recovered asa solid polymer crumb which did not exhibit cold flow.

The hydrogenated copolymer was added to oil compositions as described inExample 1. The low temperature viscosity of the oil compositions weremeasured according to ASTM D-4684 and the results are reported in thefollowing Table.

EXAMPLE 4

Living isoprene blocks were prepared by anionically polymerizing about85% of an initial charge of 56.20 lbs. of isoprene in cyclohexane. Thepolymerization reaction was promoted by sec-butyllithium in the absenceof any randomizing compounds. The living isoprene blocks had at least93% 1,4-polymerized isoprene units. Polymerization continued after theaddition of 3.89 lbs. of styrene until the number average molecularweight of the copolymer was 262,400. The copolymer had a polystyrenecontent of 6.3% and the total molecular weight of block polystyrene was9,900 as measured by GPC prior to hydrogenation according to Example 1.

The copolymer was hydrogenated as described for Example 1 although theextent of hydrogenation was not measured. The copolymer was recovered asa solid polymer crumb which did not exhibit cold flow.

The hydrogenated copolymer was added to oil compositions as described inExample 1. The low temperature viscosity of the oil compositions weremeasured according to ASTM D-4684 and the results are reported in thefollowing Table.

EXAMPLE 5

Living isoprene blocks were prepared by anionically polymerizing about85% of an initial charge of 15.44 lbs. of isoprene in cyclohexane. Thepolymerization reaction was promoted by sec-butyllithium in the absenceof any randomizing compounds. The living isoprene blocks at least 93%1,4-polymerized isoprene units. Polymerization continued after theaddition of 1.060 lbs. of styrene until the number average molecularweight of the copolymer was 204,800. The copolymer had a polystyrenecontent of 6.8% and the total molecular weight of block polystyrene was10,200 as measured by GPC prior to hydrogenation according to Example 1.

The copolymer was hydrogenated as described for Example 1. Hydrogenationreduced the polystyrene content to 5.8%. The copolymer was recovered asa solid polymer crumb which did not exhibit cold flow.

The hydrogenated copolymer was added to oil compositions as described inExample 1. The low temperature viscosity of the oil compositions weremeasured according to ASTM D-4684 and the results are reported in thefollowing Table.

EXAMPLE 6

Living isoprene blocks were prepared by anionically polymerizing about85% of an initial charge of 13.66 lbs. of isoprene in cyclohexane. Thepolymerization reaction was promoted by sec-butyllithium in the absenceof any randomizing compounds. The living isoprene blocks had at least93% 1,4-polymerized isoprene units. Polymerization continued after theaddition of 0.97 lbs. of styrene until the number average molecularweight of the copolymer was 216,400. The copolymer had a polystyrenecontent of 6.9% and the total molecular weight of block polystyrene was11,300 as measured by GPC prior to hydrogenation according to Example 1.

The copolymer was hydrogenated as described for Example 1. Hydrogenationreduced the polystyrene content to 6.5%. The copolymer was recovered asa solid polymer crumb which did not exhibit cold flow.

The hydrogenated copolymer was added to oil compositions as described inExample 1. The low temperature viscosity of the oil compositions weremeasured according to ASTM D-4684 and the results are reported in thefollowing Table.

EXAMPLE 7 (Comparison)

Living isoprene blocks were prepared by anionically polymerizing about85% of an initial charge of 18.76 lbs. of isoprene in cyclohexane. Thepolymerization reaction was promoted by sec-butyllithium in the absenceof any randomizing compounds. The living isoprene blocks had at least93% 1,4-polymerized isoprene units. Polymerization continued after theaddition of 1.28 lbs. of styrene until the number average molecularweight of the copolymer was 181,200. The copolymer had a polystyrenecontent of 5.6% and the molecular weight of block polystyrene was 7,100as measured GPC prior to hydrogenation according to Example 1.

The copolymer was hydrogenated as described for Example 1. Hydrogenationof the isoprene units also reduced the polystyrene content to 3.6%. Thecopolymer was not cyclone finishable as a polymer crumb.

The hydrogenated copolymer was added to oil compositions as described inExample 1. The low temperature viscosity of the oil compositions weremeasured according to ASTM D-4684 and the results are reported in thefollowing Table.

EXAMPLE 8 (Comparison)

Living isoprene blocks were prepared by anionically polymerizing about100% of an initial charge of 19.33 lbs. of isoprene in cyclohexane. Thepolymerization reaction was promoted by sec-butyllithium in the absenceof any randomizing compounds. The living isoprene blocks had at least93% 1,4-polymerized isoprene units. Polymerization continued after theaddition of 0.69 lbs. of styrene until the number average molecularweight of the copolymer was 180,000. The copolymer had a polystyrenecontent of 3.3% and a polystyrene block having a molecular weight of5,800 as measured by standard GPC prior to hydrogenation.

The copolymer was hydrogenated as described for Example 1. Hydrogenationreduced the polystyrene content to 2.7%. The copolymer was cyclonefinishable as a polymer crumb.

The hydrogenated copolymer was added to oil compositions as described inExample 1. The low temperature viscosity of the oil compositions weremeasured according to ASTM D-4684 and the results are reported in thefollowing Table.

EXAMPLE 9 (Comparison)

Living isoprene blocks were prepared by anionically polymerizing about100% of an initial charge of 14.04 lbs. of isoprene in cyclohexane. Thepolymerization reaction was promoted by sec-butyllithium in the absenceof any randomizing compounds. The living isoprene blocks had at least93% 1,4-polymerized isoprene units. Polymerization continued after theaddition of 0.97 lbs. of styrene until the number average molecularweight of the copolymer was 189,700. The copolymer had a polystyrenecontent of 6.4% and a polystyrene block having a molecular weight of13,300 as measured by standard GPC prior to hydrogenation.

The copolymer was hydrogenated as described for Example 1. Hydrogenationreduced the polystyrene content to 6.2%. The copolymer was recovered asa solid polymer crumb which did not exhibit cold flow.

The hydrogenated copolymer was added to oil compositions as described inExample 1. The low temperature viscosity of the oil compositions weremeasured according to ASTM D-4684 and the results are reported in thefollowing Table.

EXAMPLE 10 (Comparison)

Living isoprene blocks were prepared by anionically polymerizing about100% of an initial charge of 28.50 lbs. of isoprene in cyclohexane. Thepolymerization reaction was promoted by sec-butyllithium in the absenceof any randomizing compounds. The living isoprene blocks had at least93% 1,4-polymerized isoprene units. Polymerization continued after theaddition of 1.50 lbs. of styrene until the number average molecularweight of the copolymer was 207,000. The copolymer had a polystyrenecontent of 5.9% and a polystyrene block having a molecular wieght of14,000 as measured by standard GPC prior to hydrogenation.

The copolymer was hydrogenated as described in Example 1 although theextent of hydrogenation was not determined. The copolymer wascyclone-finishable as a polymer crumb which did not exhibit cold flow.

The hydrogenated copolymer was added to oil compositions as described inExample 1. The low temperature viscosity of the oil compositions weremeasured according to ASTM D-4684 and the results are reported in thefollowing Table.

                                      TABLE                                       __________________________________________________________________________                  Polystyrene Content          Visc., poise.sup.c                        Total MW                                                                             Initial wt %                                                                         Final wt %                                                                           Net Loss.sup.a                                                                      Polystyrene MW.sup.b                                                                   5W-30, @ -30° C.                                                                 10W-40,                                                                       @ -25°            __________________________________________________________________________                                                         C.                       Examples                                                                      1      189,100                                                                              6.5    5.1    22%    7,800   207       197                      2      201,400                                                                              6.2    5.4    13%    9,100   239       197                      3      231,000                                                                              6.4    4.5    30%   10,800   247       178                      4      262,400                                                                              6.3    --     --     9,900   232       227                      5      204,800                                                                              6.8    5.8    15%   10,200   282       238                      Comparisons                                                                   6      216,400                                                                              6.9    6.5     6%   11,300   304       242                      .sup. 7.sup.d                                                                        181,200                                                                              5.6    3.6    36%    7,100   227       189                      8      180,000.sup.e                                                                        3.3    2.7    18%    5,800   248       203                      9      189,700.sup.e                                                                        6.4    6.2     3%   13,300   310       259                      10     207,000.sup.e                                                                        5.9    --     --    14,000   --        325                      __________________________________________________________________________     .sup.a Hydrogenation increases copolymer weight giving an appearance of       about a 3% loss in polystyrene. Thus, Example 9 indicates that styrene        units were not hydrogenated.                                                  .sup.b Total blocky segments of polystyrene measured prior to                 hydrogenation by GPC after ozone degradation which excludes isolated or       terminal styrene units.                                                       .sup.c A viscosity less than 300 is acceptable if the oil compositions do     not exhibit a yield stress. Yield stresses were not exhibited.                .sup.d Not cyclonefinishable as a solid crumb                                 .sup.e Diblock having no copolymer block                                 

Examples 1-5 exemplify sufficient hydrogenation of the preferredA--A/B--B copolymers to provide both cyclone-finishability and good lowtemperature viscosity. Comparative Example 6 represents an A--A/B--Bcopolymer having insufficient hydrogenation of styrene units (about 3%)to provide good low temperature viscosity. Comparative Example 7represents an A--A/B--B copolymer having too few polystyrene segments,as a result of too many hydrogenated styrene units (about 36%), to becyclone finishable as a solid polymer.

Comparative Example 8, a sequential diblock copolymer, had a goodbalance of properties although outside the scope of the presentinvention. Polymerization of such small styrene blocks in diblockcopolymers is difficult to control in commercial processes andsubsequent hydrogenation of the isoprene presents a substantial risk ofpreparing a non-finishable polymer that must be cleaned from the processequipment. Comparing Examples 1-3 to Comparative Example 8, the additionof higher amount of styrene in an A--A/B--B copolymer instead of an A--Bcopolymer can result in cyclone-finishable copolymers which haveimproved properties.

Acceptable low temperature viscosities for modern engines are less than300 poise with lower values being preferred. Examples 1 and 3 exhibitedthe lowest viscosity for a 5W-30 oil and a 10W-40 oil with respect tothe copolymers of this invention. Comparative Example 7 yielded a lowviscosity for a 10W-40 oil, but the copolymer could not becyclone-finished as a solid polymer crumb as previously stated.Comparative Examples 6, 9, and 10 establish that larger amounts ofpolystyrene in the copolymers leads to excessive low temperatureviscosities.

While the present invention has been described and illustrated byreference to particular embodiments thereof, it will be appreciated bythose skilled in the art that the same lends itself to variations notnecessarily illustrated herein. For this reason, then, reference shouldbe made solely to the appended claims for purposes of determining thetrue scope of the present invention.

Having thus described and illustrated the present invention, what is claimed is:
 1. A cyclone-finishable polymeric viscosity index improver, produced by the process of:anionically polymerizing a copolymer comprising:a number average molecular weight between 125,000 and 275,000; from 90 to 98 percent by weight of isoprene, wherein at least 85% of the isoprene units have the 1,4-configuration; from 10 to 2 percent by weight of a monoalkenyl aromatic hydrocarbon; and a structure of A--A/B wherein A is a homopolymer block of the isoprene and A/B is a copolymer block of the isoprene and the monoalkenyl aromatic hydrocarbon, the homopolymer block comprising at least 60% of the copolymer by weight; and hydrogenating at least 98% of the isoprene and from 10% to 30% of the monoalkenyl aromatic hydrocarbon.
 2. The viscosity index improver of claim 1 wherein A is a homopolymer block of isoprene having a molecular weight between 90,000 and 225,000.
 3. The viscosity index improver of claim 1 wherein the monoalkenyl aromatic hydrocarbon is styrene.
 4. The viscosity index improver of claim 3 wherein, prior to hydrogenation, the copolymer has the structure A--A/B--B wherein the copolymer molecular weight is between 150,000 and 240,000, the isoprene content is between 93% and 97%, the styrene content is between 7% and 3%, the homopolymer block of isoprene comprises at least 75% of the copolymer by weight, and B is a homopolymer block of styrene having a number average molecular weight of at least 4,000.
 5. The viscosity index improver of claim 4 wherein at least 93% of the isoprene units have the 1,4-configuration and B is a homopolymer block of styrene having a number average molecular weight of at least 6,000.
 6. The viscosity index improver of claim 5 wherein after hydrogenation the copolymer molecular weight is between 190,000 and 210,000 and the styrene content is between 6 and 4% by weight. 